1. Field of the Invention
The present invention relates to a color thermal printing system having a color correcting unit for both the input data and the dye being used in the thermal printer.
2. Description of the Prior Art
In a color printing system of a thermal transfer recording system, the dye coated on an dye carrier film is sublimated into a recording paper by heat energy generated by exothermic resistors.
FIG. 1 is a block diagram showing an example of a control circuit in a conventional color printing thermal system of the thermal transfer recording type. In the diagram, reference numerals 1r , 1g , and 1b denote input terminals of digital image data of three colors which are R (red), G (green) and B (blue), respectively. These digital image data are written into corresponding data memories 2r , 2g and 2b under the control of a write control circuit (not shown), respectively. The data memories 2r , 2g and 2b have memory capabilities adapted to store image data for printing of one frame. Since the printing is performed on a line-by-line basis in one frame, the image data corresponding to each color includes print data relating to line. The print data on each line includes density gradation data representative of the dye density gradation or gray scale of each image pixel. The dye density of each pixel is caused by the heat produced by a particular on of n exothermic resistors (which will be explained later) at a position corresponding to each exothermic resistor.
Numeral 3 denotes a color correcting circuit for making print data on the three colors C (cyan), M (magenta) and Y (yellow) from the image data R, G and B respectively and for obtaining the gray balance or tone scale of the coloring matter in the film. The color correcting circuit includes three look-up tables 3c, 3m and 3y for color correction corresponding to the data memories 2r, 2g and 2b, respectively. As an example of the function of the look-up tables 3c, 3m and 3y, these tables make the transfer data on C, M and Y by obtaining the reciprocal numbers of the respective input data. For example, cyan dye will absorb red light and pass blue and green light. For a high dye density cyan pixel little or not reflected red light will be seen by an observer. For a low dye density cyan pixel, almost all of the red light well be seen by an observer. Thus, the cyan dye modulates red light. Similarly, the yellow dye modulates blue light and the magenta dye modulates green light.
Numeral 4 denotes a switching circuit having fixed contacts 4c, 4m and 4y which are respectively connected to the look-up tables 3c, 3m and 3y, and a movable contact 4o. The output from the switching circuit 4 is supplied to one input of a data comparator 5 and an output of a data counter 6 is supplied to the other input. The output of the data counter 6 is increased one by one each time a clock signal is received from a read control circuit 7 to control the readout of the print data from the data memories 2r, 2g and 2b and the switching of the contacts of the switching circuit 4.
An output signal DA of the data comparator 5 is stored in a shift register 9 in a thermal transfer print head TH under the control of a timing control circuit 8. The content of the shift register 9 is transferred to a latch circuit 10 when a latch signal LA is supplied from the timing control circuit 8. When gates 11 are opened by an enable signal EN from the timing control circuit 8, the signals from the latch circuit 10 are respectively applied to exothermic resistors 12.
The operation of the above mentioned prior system will now be described with reference to FIG. 1.
When a start signal is output from the timing control circuit 8, the read control circuit 7 performs control so that the movable contact 4o of the switching circuit is connected to the fixed contact 4c and also send an address signal to the data memory 2r, thereby allowing the first line print data to be read out. This print data is supplied to the data comparator 5 through the look-up table (LUT) 3c and the contact 4c and 4o. At the same time, the read control circuit 7 sends a clock signal to the data counter 6, thereby allowing "1" to be output therefrom and input to the data comparator 5. The data comparator 5 individually compares each density gradation data in the print data of the first line with "1" as the out put from the data counter 6. If the density gradation data is equal to or larger than "1", a signal representing "1" is output from the comparator 5. In the case of the density gradation data being lower than "1", a signal representing "o" is output. Namely, the data comparator 5 outputs "1" when the density gradation data is equal to or larger than "1", and outputs "0" in other cases. Based on the control performed by a clock signal CL from the timing control circuit 7, the output DA of the data comparator 5 is set in the shift register 9 is the thermal print head apparatus TH, which position corresponds to the order of the print data. The timing control circuit 8 subsequently sends the latch signal LA to the latch circuit 10, thereby allowing the data in the shift register 9 to be latched into the latch circuit 10. At this time, since the timing control circuit 8 has already applied an enable signal EN to the gates 11, only those exotermic resistors corresponding to the bits in which are holding "1" are selectively energized and generate heat.
Next, the read control circuit 7 again reads out the print data of the first line from the data memory 2r and controls the data counter 6 to output "2" in this case. When the density gradation data on the print data is equal to or larger than "2", the data comparator 5 outputs "1". On the other hand, when it is smaller than "2", the data comparator 5 outputs "0". These outputs are set into the bits of the shift register 9 respectively. The exothermic resistors corresponding to the bits which are set as "1" are driven in a manner similar to that mentioned above. The operations similar to those mentioned above are repeated hereinafter until the output from the data counter 6 is equal to the maximum value in the density gradation values. The printing of the first line if finished in this manner.
After completion of the transfer of the first line, the print data of the second and subsequent lines are sequentially read out of the data memory 2r in a similar manner and the printing of each line is thus performed. In this manner, the C-printing of one frame is finished. After completion of the C-printing, the read control circuit 7 controls the movable contact 4o of the switching circuit 4 to connect it to the fixed contact 4m. In a manner similar to the case of the C-printing, the M-printing of one frame by the print data of M is performed to transfer magenta dye (M) into the receiver which already received the cyan dye (C). Thereafter, the Y-printing (yellow) of one frame is executed by the print data of Y so as to transfer the yellow dye into the receiver. Thus, one color image is formed in a recording medium such as paper in the manner described above.
In the foregoing description, although the switching circuit 4 has merely been shown in the form of mechanical switch for the convenience of explanation, obviously electronic switches can also be used. In addition, although the look-up tables 3c, 3m and 3y are provided after the data memories 2r, 2g, and 2b in FIG. 1, this order may also be reversed. After the image data R, G and B have been color corrected by the look-up tables, they may be stored in the data memories. For a more detailed description of a thermal printer reference may be made to U.S. Pat. No. 4,710,783 to Caine et al.
In many cases, the conventional color printing system of a thermal transfer recording system receives data from a single image data source (e.g., TV camera to output image data of three colors R, G and B, or the like); therefore, no problem occur.
However, when there are a plurality of image data sources, for example, in a case where the source is selected from one of the TV signal from a television receiver, image signal from a TV camera, computer graphics data, image data from a document scanner and the like, and the signal of the selected source is supplied to a control circuit of a color printing system of a thermal transfer recording system, the color characteristics differ as between the respective image data sources. Moreover, in general, the color characteristics of dye carrier films or dye donors also differ according to the manufacturer. Therefore, in the conventional color printing systems of a thermal transfer recording system in which the color correcting circuit is effective only with respect to a single image data source, there is a problem in that it is difficult to perform accurate color correction when signals are supplied from a plurality of image data sources or when dye films of different manufacturers are used.